Ctor.toString() breaks on environments where Reflect.ownKeys is undefined (e.g. node v4.8.1)

[pateketrueke]

Version

2.2.6

Reproduction link

https://gist.github.com/pateketrueke/a22ef3b406372efad46cdab74b25727a

Steps to reproduce

I found this bug loading vue-template-compiler@2.2.6 under node v4.8.1 on TravisCI, no steps are required, see the gist.

What is expected?

Get vue-template-compiler running without errors

What is actually happening?

Loading vue-template-compiler breaks my tests

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